There are numerous instances where the width of a belt, strip or other generally laminar work (hereafter "material" for brevity) needs to be monitored, not so much to determine what the precise width of the material actually is, but to position the material properly for some specific purpose. Guiding and positioning material is especially important if it is to be cut or otherwise treated. For instance, before carpeting and wall paper coverings are rolled for shipment, they are cut into predetermined widths in devices such as are disclosed in U.S. Pat. Nos. 2,910,122 and 3,073,197; in another apparatus described in U.S. Pat. No. 3,614,445 beta rays are used to determine width variations of films or strips of synthetic resinous materials which are edge-trimmed before they are formed into bags; and, various mechanical and optical means are used in devices in which excess rubber forming at edges of calendered fabric is trimmed; or, rolls of calendered rubberized material are slit into continuous belts of predetermined width before they are used to build an automobile tire. It is in the field of building tires that this invention is of particular interest.
The invention comprises an apparatus which includes at least one active edge-sensing means (referred to herein as a "sensor") for sensing at least one edge of a material, a suitable electric circuit means to provide an output signal derived from current generated in the sensor, and positioning means for positioning the material, which positioning means are controlled by a drive means as a direct result of the receipt by said drive means of the output signal. The sensor comprises (i) a silicon solar cell (also referred to as an "eye" or "cell") which generates current upon exposure to light (ii) a light source ("lamp") to provide the light, and (iii) a suitable electrical circuit means to measure the magnitude of a transmitted current generated within the cell when a portion of it is exposed to light.
Although the characteristic properties of a solar cell are well known, these characteristics have never been successfully utilized to provide the specific function of positioning a material by sensing at least one of its edges, by those persons who needed such a system. Moreover, nothing about these characteristics suggested that extreme accuracy could be obtained with so reliably rugged and compellingly simple a system.
Photoresistors, and photopotentiometers or photobridges are photoelectric sensors which have been disclosed in many devices used in the field of monitoring widths of material for different purposes. These devices are "passive" devices and none of these prior art devices utilizes a solar cell which is unique in that it alone generates an electric current when it is exposed to light. Moreover the response of a solar cell is unique in that it is essentially linear. By "linear" is meant that a unit increment of current generated in the cell for each unit increment of light falling upon it, is constant. Stated differently, the amount of current generated in the cell, is directly proportional to the area of the cell exposed to the light. Since the light falling upon the cell is proportional to the light intensity, it is critical that the light be substantially uniform per unit of area. In order to obtain a constant overall light intensity, a feedback circuit may be used to adjust the lamp current. In an alternative configuration, the light intensity is permitted to vary and a feedback circuit is used to modify the circuit. This assures constant signal per unit area of cell exposed to the light.
In an article captioned "Analog Sensing of Displacement Using Silicon Solar Cell", by Haruo Shinatani, in IEEF Transactions on Industrial Electronics and Control Instrumentation, Vol. IECI-26, No.2, May 1979, a silicon solar cell is used for an optical sensing displacement in an analog form, and an application to a pressure gauge is given. The sensor consists of a set of lamp and cell with a lamp-control circuit, where a shutter interrupts the light beam. The principle of operation, linearity, temperature effect, frequency response, and associated circuit design are presented in a description which is incorporated by reference as if fully set forth herein. From the discussion of linearity, it will now be evident that prior art devices using sensors which do not include a solar cell, are not linear but exponentially responsive to light falling upon them.